Exploring a lightweight Mobile Hab based HTM architecture

[randomly]

Using the same amount of Pu238 as the Cassini mission in a sterling generator would give you about 4000 watts electric from the 13200 watts thermal. Obviously this amount of heat can be accommodated in a launch as it's been done. The sterling generator weighs only 1.2 Kg out of the total 20 Kg ASRG, 0.8 Kg of Pu238, the rest is casing and radiator fins. Scaling up the ASRG to higher power will definitely give a considerably better power to weight ratio if all that plutonium in the Cassini mission was accomodated in 168 Kg of RTG assembly.

Also if you use some of the waste heat to heat your hab you can minimize the amount of hab insulation required and control your hab temperature by balancing the heat losses with variable amounts of waste heat you pump into it. So not only does the radioisotope system provide most or all of your electrical power, it can provide all the necessary heating needed as a freebee (power wise anyway).

[Kaputnik]

Using the same amount of Pu238 as the Cassini mission in a sterling generator would give you about 4000 watts electric from the 13200 watts thermal. Obviously this amount of heat can be accommodated in a launch as it's been done.

Am I right in thinking Cassini's RTGs are cooled entirely by passive radiation via the fins? The spacecraft was only cooped up in a PLF for a relatively short while during launch- RTGs on a mars payload would be enclosed for the entire interplanetary cruise phase. I have anecdotally heard that this is a major problem for inclusion of very large RTGs on Mars missions.
I suppose it could be feasible to include an active cooling system with radiators which is discarded along with the cruise bus prior to entry- but it will still add mass to the entry vehicle due to the necessary parts of the cooling system which must be within the aeroshell.

Another point, from a quick look at Mike's mass breakdowns- 100kg for batteries. Really!? What can Li-Po or Li-ion batteries store, per kg?

[Michael Bloxham]

It doesn't matter. If we assume it is possible to schedule power demands around available solar power (i.e. only driving during the middle part of the day, running other processes around what power is left over, etc.), and perhaps a small ~0.5kW RTG, then we don't need a large battery. A battery becomes a nice-to-have - perhaps only a very small one may be useful as a means to ease power management. Getting rid of the need for a large battery means more mass is left over for the stuff that actually *produces* power - so the average power output is higher for a given mass allocation.

[MikeAtkinson]

The technology roadmap has batteries at 500Wh/kg for general use (up to 1000Wh/kg for deep space probes) and up to 2700Wh/kg for advanced flywheel systems.

[MikeAtkinson]

Mike, I've just looked at your spreadsheet. A few things jump out at me

1. you seem to have only a floor area of abut 7m2, is this really enough for 2 people for up to 200 days?

2. Its hard to estimate the volume of the consumerables (and their containers) and internal equipment and fittings, but I would guess 10-20m3.

3. you don't seem to have made any allowance for spares or tools.

4. the chassis has to cope with the total mass of the vehicle, are you sure 200 kg is going to be enough (I assume this includes the wheels and drive motors as well).

5. there doesn't seem to be enough allowance for CO2 removal, how do you plan to do that by the way?

6. I gather that they would operate in pairs, what happens if one breaks down, are the systems sized to cope with four crew for any period of time?

When I looked at designing a long duration rover many years ago (up to 60 days) now I used LOX/methane, it meant having to pull a trailer but that gave the best energy density in a movable form. Running it through a fuel cell gives water which greatly reduces the amount of pottable water needed.

The things I found hardest were dust control (how do you allocate enough volume for an air lock were the crew can get in and out of their space suits and clean and repair them), and emergency situations (crew rescue, breakdowns, etc.).

Thermal management was assumed to be passive, there was enough heat being generated in the pressure shell to maintain a reasonable temperature.

[MickQ]

Mike, I've just looked at your spreadsheet. A few things jump out at me

1. you seem to have only a floor area of abut 7m2, is this really enough for 2 people for up to 200 days?

2. Its hard to estimate the volume of the consumerables (and their containers) and internal equipment and fittings, but I would guess 10-20m3.

3. you don't seem to have made any allowance for spares or tools.

4. the chassis has to cope with the total mass of the vehicle, are you sure 200 kg is going to be enough (I assume this includes the wheels and drive motors as well).

5. there doesn't seem to be enough allowance for CO2 removal, how do you plan to do that by the way?

6. I gather that they would operate in pairs, what happens if one breaks down, are the systems sized to cope with four crew for any period of time?

When I looked at designing a long duration rover many years ago (up to 60 days) now I used LOX/methane, it meant having to pull a trailer but that gave the best energy density in a movable form. Running it through a fuel cell gives water which greatly reduces the amount of pottable water needed.

The things I found hardest were dust control (how do you allocate enough volume for an air lock were the crew can get in and out of their space suits and clean and repair them), and emergency situations (crew rescue, breakdowns, etc.).

Thermal management was assumed to be passive, there was enough heat being generated in the pressure shell to maintain a reasonable temperature.

Speaking of dust control, has anyone considered the NASA LER (?) design where the suit attaches to the outside of the vehicle and the crew access is thru a hatch in the back of the suit.  An airlock would still be needed for emergency use but by keeping dirty suits outside for normal day to day ops then much less dust should make it inside.

Mick.

[Michael Bloxham]

Mike, I've just looked at your spreadsheet. A few things jump out at me

Thanks Mike.

1. My figures show ~16m^2 (~8m^2 of floor space for each crew member). Whether this is "enough" - I am not one to make the final judgement. But at 25m^3 per crew member it is in line with the recommendations in Larson & Pranke for a ~6 month voyage. On the surface the internal volume of the "ASH" also becomes available (another ~50m^3 which can be shared by all four crew members), and the crew can also occasionally swap Mabs, and also have ample EVA opportunities, etc.
25m^2 is more than that of a large terrestrial motor-home.

2. In the excel spreadsheet I attached, I have assumed that the total food requirement is spread among the surface vehicles - so that the ASH carries 1200kg, the ISRU/MAV another 1200kg, and the MABs land with 600kg each - enough for the first 200 days. Each MAB would also have another 600kg on top of this for the ~200 day interplanetary voyage - bringing the total intial load to 1200kg each also. Assuming an average density of 0.3kg/L (including packaging) this would take up about 3.6m^3 of volume.

I have no idea about the volume requirements of internal equipment, provisions, etc. I guess it depends also on what systems / equipment needs to reside within the pressurized volume, and which can be attached to the outside - as there is quite a bit of extra volume that could be used here.

[Michael Bloxham]

3. No - you're right. I'm not an expert here so I have no idea what figure is prudent. One of our other members has more expertise in this area. Hopefully we can put a bit more analysis into things and narrow down what we need. Perhaps the figure needn't be unmanagably large - if emphasis is put on making the "big important things" ultra-reliable: grey-water recycling, passive heat management, solar power, etc. Rather than relying on less robust or more risky technologies.

4. The mass provision for a "chassis" - which in my mind would constitute localized reinforcements to the main pressure shell - so that it acted as a single rigid monocoque - is 400kg - which is half that of the pressure shell itself (800kg). That's just a guess, of course. There is a separate mass provision for the suspension system, wheels, motors, etc. The total mass of the "undercarriage", including the "chassis", is 1340kg.

5. I have no idea (not an expert here) but there is a 500kg mass provision for this kind of stuff - which should be ample for a crew of two.

6. Yes.

[Michael Bloxham]

When I looked at designing a long duration rover many years ago (up to 60 days) now I used LOX/methane, it meant having to pull a trailer but that gave the best energy density in a movable form. Running it through a fuel cell gives water which greatly reduces the amount of pottable water needed.

The things I found hardest were dust control (how do you allocate enough volume for an air lock were the crew can get in and out of their space suits and clean and repair them), and emergency situations (crew rescue, breakdowns, etc.).

Thermal management was assumed to be passive, there was enough heat being generated in the pressure shell to maintain a reasonable temperature.

Sounds interesting! You seem to have a little experience in these things. Perhaps you would you like to help us?

Methane/LOX propulsion is still an attractive option, IMHO, for the reasons you mentiond (free fresh water) and others (capitalize ISRU and not susceptable to prolonged low-light conditions). Perhaps there is the potential for a methane fuel cell / solar hybrid, in combination and also with a very small RTG perhaps?

In regards to dust-control: Perhaps the aforementioned ASH (basically a double-ended MobileHab which travels along with the crew) could act somewhat as a "portable lab": allowing intense sample analysis and also suit repair. Gearing the ASH towards exclusively surface-based functions and facilities makes sense because it it the only vehicle which is used exclusively on the surface.

[Michael Bloxham]

Speaking of dust control, has anyone considered the NASA LER (?) design where the suit attaches to the outside of the vehicle and the crew access is thru a hatch in the back of the suit.  An airlock would still be needed for emergency use but by keeping dirty suits outside for normal day to day ops then much less dust should make it inside.

Mick.

Yes - I like this idea. Perhaps there could be such "suit-ports" on each MAB, while the ASH would contain the large air-lock (or perhaps *be* the large air-lock). I wonder whether the internal volume of each MAB would be small enough to allow the whole MAB itself to function as an emergency air-lock? In MP4, it is assumed that the O2 is not recycled - so each MAB would carry a few hundred kg of the stuff. Perhaps the MAB could be quickly vented in an emergency - a door opened up - and the atmosphere quickly restored via the supply of onboard O2 - with buffer-gas gradually reintroduced sometime after?

[randomly]

The technology roadmap has batteries at 500Wh/kg for general use (up to 1000Wh/kg for deep space probes) and up to 2700Wh/kg for advanced flywheel systems.

These numbers sound highly optimistic. As a real world example the Mars Exploration rovers used two 28Volt 10 amp/hour Lithium/sulfur dioxide batteries. Lithium Sulfur dioxide is rated 250 Wh/kg but the actual finished battery assemblies in the MERs only achieved about 50 Wh/kg. One must be careful about the theoretical energy density figures quoted versus the energy density achieved with actual batteries. The highest energy density chemistries may not always be indicated because of the need to operate at temperature extremes, requirements for a large number of charge/discharge cycles, or operation at high current densities. At this point in time I would think 100 Wh/kg is a fairly safe bet.

The advanced flywheel systems sound very interesting though. If they can be made rugged enough and reliable enough. I have no idea if you can make magnetic bearings suitable for reliable operation in a vehicle in motion across rough ground. This might limit the energy density you can achieve for this application.

[MickQ]

Regarding power supply.  If ISRU successfully produces Methane/Oxygen, could the gasses be used to run portable combustion generators for temporary, emergency or remote power applications ?

Mick.

[Michael Bloxham]

Regarding power supply.  If ISRU successfully produces Methane/Oxygen, could the gasses be used to run portable combustion generators for temporary, emergency or remote power applications ?

Mick.

Perhaps, but you might need a small solar array and cryo-cooler to keep these cryogenics cool enough to prevent excessive boil-off. While the Martian atmosphere is very light, it is still pervasive - and will penetrate conventional MLI; allowing a significant amount of conduction / convection over time. For constant-use applications (such as providing power for a mobile-hab) the situation isn't so bad - as the boil-off can be consumed as it is generated. But I would suggest that it is less useful for delayed-use applications.

[Michael Bloxham]

The advanced flywheel systems sound very interesting though. If they can be made rugged enough and reliable enough. I have no idea if you can make magnetic bearings suitable for reliable operation in a vehicle in motion across rough ground. This might limit the energy density you can achieve for this application.

I know the KERS system used in Formula One works quite well - and is subject to much much higher shock loads than a MobileHab roving at ~3kmh would ever experience.

[MikeAtkinson]

Sounds interesting! You seem to have a little experience in these things. Perhaps you would you like to help us?

No, for three reasons:

1. been there, done that.
2. I don't have the time
3. Paper studies quickly reach a point where there are too many unknowns to make further progress.

I think you should be looking at making a space model. Build something the correct size and fit it out with floor, tanks, storage lockers, seats, air lock, etc. and see whether it is big enough.